1. Field of the Invention
The present invention relates to an ink jet print head that ejects ink according to an ink jet method, and also relates to an ink jet printing apparatus that performs printing on a print medium by using the ink jet print head. In particular, the present invention relates to a technique to reduce the generation of air current at the time of the ejecting operation of an ink jet print head that includes an array of plural ejecting opening columns.
2. Description of the Related Art
High-speed output, high-resolution printing, high quality of image, and low noise are some of the properties that are required for the various types of printing apparatuses having recently been developed. Ink jet printing apparatuses are examples of the printing apparatuses that can satisfy the above-mentioned requirements. In the ink jet printing apparatus, ink (printing liquid) drops are ejected from ejecting openings formed in the print head, and made to fly. Then the ink drop is attached on a print medium to form a dot at predetermined positions.
The ink jet printing apparatus is provided with means for generating energy to eject ink. An electrothermal transducing element such as a heater and a piezoelectric element are some of the examples of the above-mentioned energy generating means. Applying voltage to an electrothermal transducing element generates heat rapidly in the electrothermal transducing element to cause film boiling of the ink located nearby. The phase transition of the ink causes foam pressure, which makes the ink ejected, as drops, from the ejecting openings. On the other hand, applying voltage to a piezoelectric element causes a deformation of the piezoelectric element. Pressure generated at the time of the deformation makes the ink ejected, as drops, from the ejecting openings.
Incidentally, increasing demands for higher-speed and higher image quality of printing have caused changes related to the recent ink jet printing apparatuses. Apparatuses have now been developed that have an increased number or density of ejecting openings arrayed in the printing head, a reduced size of the ink drops, and an increased ejection frequency. Now, suppose a case where printing is performed by ejecting ink at high frequency from a printing head with a large number of ejecting openings that are densely arrayed. It is known that, in this case, multiple ink drops ejected at high speed sometimes cause air currents between the print head and the print medium, and that such air currents affect the direction in which the ink drops fly.
FIG. 9 is a schematic diagram for describing a case where the air currents affect the direction in which the ink is ejected. While a print head 100 shown in FIG. 9 moves, relative to a print medium P, in the main-scanning direction indicated in FIG. 9 at a predetermined speed, the print head 100 ejects ink drops 300 from ejecting opening columns 201 and 202 to the print medium P at a predetermined frequency. Each of the ejecting opening columns 201 and 202 includes an array of plural ejecting openings arranged in the vertical direction in the drawing. The ink drops ejected from the ejecting opening columns 201 and 202 at high speed and high frequency generate air currents 11 near the ejecting opening columns 201 and 202. The air currents 11 thus generated interfere with one another, which deflects the advancing direction of the ink drops 300 that would otherwise have been directed perpendicularly to the print medium P. Consequently, dots are printed on the print medium P at positions that are different from their respective originally-targeted positions. The degree of such deflection depends on the magnitude of the air currents, which in turn depends on the actual ejection frequency of the ink ejected from the individual ejecting opening columns 201 and 202, that is, on the data for the printing. Accordingly, the amount of shifting of the dots varies depending on the data for the printing. In the outputted image, the variable amount of shifting causes such recognizable image defects as unevenness in the density.
U.S. Pat. No. 6,997,538 and U.S. Pat. No. 6,719,398 disclose print heads that blow out gas as the ink is being ejected for the purpose of reducing the harmful effects of the above-described air currents on the outputted image.
FIGS. 10, and 11A to 11C are diagrams for describing the blowing out of gas at the time of printing disclosed either in U.S. Pat. No. 6,997,538 or U.S. Pat. No. 6,719,398. These documents explain that the air currents that deflect the ejecting direction of the ink are caused by the kinetic energy of the ink ejected at high frequency and at high speed as well as by the movement of the carriage at the time of printing. FIG. 10 illustrates an exemplar configuration to reduce the air currents. In the configuration, a gas blowing-out opening 70 is provided at the front side of the carriage in the direction in which the carriage is advancing. At the time of printing, the gas is blown out in a direction which is perpendicular to the ejecting direction of the ink and which is parallel with the scanning direction of the carriage. However, when plural ejecting opening columns are arranged side by side with one another in the advancing direction of the carriage, the effects obtained by the blowing out of the gas in the configuration of FIG. 10 may possibly differ among the plural ejecting opening columns. Specifically, the stream of the gas blown out is strong around the ejecting opening column located closer to the gas blowing-out opening 70, so that large effects of the blowing out of the gas can be expected. However, the stream of the gas blown out is weak around the ejecting opening column located farther away from the gas blowing-out opening 70, so that only small effects of the blowing out of the gas can be expected. It is certainly conceivable that a larger blowing-out power for the gas is employed in accordance with the necessity of affecting the ejecting opening column that is located farthest away from the gas blowing-out opening 70. In this case, however, the stream of the gas blown out with such a large blowing power may possibly affect, negatively, the ejecting direction of the ink from the ejecting opening columns located closer to the gas blowing-out opening 70.
In contrast to the configuration of FIG. 10, the configuration shown in FIGS. 11A to 11C includes a gas-introduction opening 90 and gas blowing-out openings 71 that are so arranged that the gas is blown out in a direction which is perpendicular to the ejecting direction of the ink and which is parallel to the ejecting opening columns. Multiple gas blowing-out openings 71 are provided at their respective positions each of which is located between two adjacent ones of the ejecting opening columns in the configuration shown in FIGS. 11A to 11C. Accordingly, even when the configuration includes multiple ejecting opening columns, the uneven effects on the plural ejecting opening columns can be avoided.
Both of the above-mentioned Patent Documents describe that the configuration to blow out the gas in a direction perpendicular to the ejecting direction of the ink makes it possible to reduce the air currents that are likely to deflect the ejecting direction of the ink.
Examination conducted by the inventors of the present invention has revealed that a gas blown out in a direction that is parallel with the ejecting direction of the ink, in some cases, stabilizes the ejecting direction of the ink better than a gas blown out in a direction that is perpendicular to the ejecting direction of the ink. In such cases, sufficient stabilizing effects on the ejecting direction of the ink cannot be obtained by a configuration in which the gas is blown out only in a direction that is perpendicular to the ejecting direction of the ink as disclosed in U.S. Pat. No. 6,997,538 or U.S. Pat. No. 6,719,398, and thus no satisfactory improvement in the problem of dot shifting can be observed.